Space telecommunication antennas, with circular polarization, often use waveguide structures when a severe polarization purity criterion is specified. Among them, the polarization system with four quadrature-fed ports, the Septum-type polarizer and the orthomode tee coupled to a screw polarizer remain the most used. However the size and mass of these systems hamper their use in certain applications and in particular in the low bands of telecommunication frequencies (L, S, C bands).
Intermodulation at the antenna level is a problem that has been known to constructors of communication satellites for many years. This problem also exists in the case of GSM antennas (the abbreviation standing for Global System for Mobile).
In the case of terrestrial satellite stations, the use of certain bands in multicarrier mode requires the solution of technical problems related to the metallic contacts appearing when making the antenna and source. Among the technical problems are adjustments by screws/plungers, interfaces of flange type, add-on parts. It is known from the prior art that one of the better approaches is to make a minimum of parts and to use electroforming technology for the construction of the microwave sources. One of the constraints to be complied with is then that the electrical design of the microwave source be adapted to enable it to be made in this particular process.
Another drawback encountered in the field of microwave structures is that they lead to relatively significant sizes of device. Thus, a solution given in the patent is to manufacture an assembly formed of an antenna and of an assembly of horns.
U.S. Pat. No. 6,861,997 describes a polarizer intended to be used in antennas associated with waveguides. The idea implemented in this patent consists in using two waveguides having a common wall. The form of the central wall consists of several teeth whose form and dimensions enable the septum to transform the linear polarization of a wave into a circular polarization and vice versa.
Nevertheless, the prior art devices generally consist in linking 2 rectangular guides to form a single guide with square cross section by way of a plate generally cut stepwise. This process works well if the plate is very slender, such as for example in U.S. Pat. No. 5,305,001, for which the plate has a thickness of the order of 0.76 mm.
FIG. 1 represents a schematic of a satellite communication station with X band circular polarization. The emission signal passes through an emission amplifier 1, then into an emission filter 2 before the polarizing orthomode 3 whose function is to transform the initial polarization, for example a linear polarization, into a circular polarization, and is then emitted by way of the horn 4 of the reflector-type antenna. The signal is thereafter received by the antenna before passing through the polarizing orthomode 3, then a reception filter 5 and a low noise amplifier 6. Practice shows that filtering is necessary but not sufficient to eliminate the intermodulation problems encountered with microwaves. Specifically, other non-linearities are present along the chain.
They originate notably from all the contacts between metals introduced by assembling the source to the focus of the antenna. These contacts are, for example, the flanges of the joining guides, the screw plungers for adjusting the filters.
When the frequency plan used in a satellite communication system comprises very close reception and emission frequencies, for example the interval 7.25-7.75 GHz for reception and the interval 7.9-8.4 GHz in emission, for example when a station emits several carriers (from 2 carriers, for example), additional frequencies may be generated. For frequencies lying between 8.0 GHz and 8.4 GHz, any non-linearity of the transmission system will create additional frequencies, the most powerful of which are 7.6 and 8.8 GHz. The 7.6 GHz frequency is located in the reception band, and this will end up being particularly demanding on the quality of linearity of the system so as not to generate self-jamming.
The software known to the person skilled in the art, for example the software having the trademark MICIAN for microwavewizard, makes it possible to synthesize and to simulate various structures.
To the Applicant's knowledge, the performance levels obtained by the devices according to the prior art nevertheless do not make it possible to maintain the desired performance levels while increasing the thickness of the plate beyond 2 mm.